recap

49.5 recap

The delivery of blood to tissues is controlled locally by autoregulatory mechanisms that dilate or constrict arterioles. These local actions are translated into alterations in central blood pressure and composition that are detected by neural and hormonal mechanisms, which then mediate corrective cardiovascular adjustments.

learning outcomes

You should be able to:

  • Describe mechanisms underlying autoregulation of local blood flow.

  • Describe the role of the autonomic nervous system in regulating MAP.

  • Draw a diagram showing how an increase or a decrease in the firing rate of baroreceptors causes a change in MAP.

  • Apply knowledge about local and systemic controls of cardiovascular functions to describe interactions between the two.

Question 1

Why is autoregulation in Figure 49.19 described as a positive feedback loop?

Figure 49.19 shows that a drop in blood flow to a tissue due to a fall in MAP results in autoregulatory responses that decrease the resistance to flow in that tissue. Any decreased resistance in the systemic circuit will cause a drop in MAP, further depriving that tissue of O2 and nutrients and increasing the autoregulatory response. With this positive feedback loop, the autoregulatory response can worsen the problem that initiated it.

Question 2

Distinguish between the roles of the two divisions of the autonomic nervous system (ANS) in regulating MAP.

The sympathetic division of the ANS stimulates vasoconstriction in tissues not essential for a fight-or-flight response, such as the digestive system and the skin. This increase in total peripheral resistance results in a rise in MAP. The sympathetic innervation of the heart causes it to beat faster and more strongly, also contributing to a rise in MAP. The parasympathetic division of the ANS has opposite effects, slowing the heart and decreasing MAP.

Question 3

Baroreceptor neurons have a range of firing rates depending on their degree of stretch, and at rest they are firing at the midpoint of that range. Why is this adaptive?

Baroreceptors are firing at a rate midpoint in their range when blood pressure is normal. Therefore they have the potential to either decrease or increase their firing rate, depending on whether MAP falls or rises. They therefore contribute to regulatory responses to both rises and falls in MAP.

Question 4

Explain how local autoregulatory controls of blood flow can stimulate hormonal adjustments of MAP.

Autoregulatory mechanisms operate at the local level to dilate or constrict arterioles in response to local changes in the chemical environment. These changes in arteriole diameter affect the amount of blood reaching local tissues. As these changes occur, they have an effect on the body’s blood pressure, which is detected by baroreceptors that elicit neural and hormonal signals that cause widespread cardiovascular changes throughout the body.